EL-FASHIR, SUDAN
— The African Union patrol was only seven miles from Sirba, the site of one of the latest Darfur massacres, when they were forced to turn back. Nearly 400 Arab militiamen in Sudanese government uniforms, with new Land Cruisers and weapons, blocked the dusty track.

Tuesday's incident was only the latest in a crackdown on access for international observers, journalists, and humanitarian organizations – a pattern that is becoming wearily familiar to those working in Darfur.

"The timing is no coincidence," says Leslie Lefkow of Human Rights Watch. "[Sudan is] stemming the flow of information from Darfur while it continues to commit massive crimes and run a military campaign."

As outgoing UN chief Kofi Annan began a major push to stem the escalating crisis during high-level meetings in Addis Ababa, Ethiopia, Thursday, the Sudanese government told top UN humanitarian official Jan Egeland that all his proposed destinations on a three-day trip to Darfur are too insecure to visit this weekend.

Last week, the Norwegian Refugee Council announced it was being forced out of Darfur after its permit to operate had been indefinitely suspended for the fifth time, making working conditions "impossible." Other foreign aid workers say they have been denied permission to reenter the country after leaving to attend a family emergency or to seek medical treatment.

Killings mount

Thirty villagers were reported killed this week in Sirba, but no outside investigators have been able to enter the town to confirm the reports.

Sudanese rebels accused government troops and militias Thursday of killing more than 50 people in another attack. Two weeks ago, 63 people were reported killed in Jebel Moon, and their bodies buried in the desert.

In that case, investigators were able to access the massacre site, and found that more than 20 of the victims were children. Some of them had been shot through the head. Survivors described Arab men in uniforms, with Thuraya satellite phones, new vehicles, and animals, similar to the group seen only a few miles away barring the road to Sirba.

The government-backed militias have been blamed for hundreds of atrocities since local, non-Arab tribes first took up arms to protest government neglect and the arming of Arab tribes three years ago. Since then, an estimated 200,000 people have died and 2.5 million have fled their homes.

After the government signed a peace deal with one of the three rebel factions last May, the militias, known as the janjaweed, were supposed to be disarmed. Instead, the government appears to be using them as a proxy force to avoid accusations of cease-fire violations. But accurate reporting of militia movements, and alleged massacres, is becoming increasingly difficult.

Crackdown on journalists, aid workers

Journalists able to secure a visa face a bewildering array of permits and paperwork; the Sudanese government must be informed in advance of any travel in Darfur. Officials insist on listening to interviews; they intimidate interviewees, and have attempted to confiscate notebooks.

"I can take any of [your permits] I want ... you're going to hell," one official hissed at this reporter. "Do you think this is a free country?" Last week, all permits for journalists to travel to the region were being denied.

The African Union (AU) monitoring force of nearly 7,000 soldiers is also frequently stymied in its investigative attempts. Officials say fuel is stolen, government permission for them to leave their bases is refused, and their soldiers have been killed when convoys were attacked.

During the one-day talks in Ethiopia with UN, EU, and Arab League officials Thursday, Mr. Annan pushed for a "hybrid" force of AU and UN peacekeepers to be allowed into Darfur. But early indications were that Sudan would reject this.

Even humanitarian organizations, charged with delivering food, water, and medicine to destitute Sudanese, are under attack. When Doctors Without Borders spoke out about rape cases last year, its two most senior staff were arrested. Since then, aid agencies say the situation has worsened.

"It's disgusting. They are holding their own people hostage to shut us up," complained one aid worker bitterly. "If we speak out, we get thrown out. Then who will help these people?"

Tens of millions dollars and a Herculean effort have managed to stabilize key humanitarian indicators like the infant mortality and malnutrition rates, and aid agencies are reluctant to risk their hard-won gains by criticizing the government for atrocities.

All aid workers interviewed for this article agreed to speak only on condition of anonymity.

But it's not only the government that is piling on the pressure. Thirteen humanitarian workers have been killed since last May's peace deal, which pitted signatory and non-signatory rebel groups against each other. As the fighting intensifies along the Chadian border and in the north, all sides in this conflict want vehicles and the shiny white 4x4s used by aid groups are a tempting target.

Carjackings of both aid vehicles and commercial trucks are rampant. In a single day last month, nine vehicles were snatched. Convoys have also been attacked and employees beaten up or sexually harassed, say aid workers here.

The breakdown in security means vast swaths of northern Darfur are no-go areas for groups providing food, medicine, and clean water for tens of thousands of displaced villagers. "It's become very dangerous to work here," says another aid worker, whose organization has suffered repeated attacks. "Before, we could negotiate with the rebels for safe access, but now we don't know who controls what territory any more."

Pinned on the wall behind her was a series of maps showing humanitarian access for Darfur month by month. The inaccessible areas, colored orange, are spreading like spilled ink.